Unlocking the electronic genome of halogenobenzenes.

The drive to develop new organic materials for use in optoelectronic devices has created the need to understand the fundamental role functionalization plays concerning the electronic properties of conjugated molecules. Here density functional theory (DFT) is used to investigate how the HOMO-LUMO gaps of halogenobenzenes are affected as a function of substituent size, position, electronegativity, ionization potential, and polarizability. A detailed molecular orbital analysis is also provided. It is shown that the molecular static polarizability and ionization potential of the bound halogens are the primary physical descriptors governing the HOMO-LUMO gap within halogenobenzenes. Two secondary descriptors controlling the HOMO-LUMO gap in these materials are the aromaticity of the halogen substituted benzene rings (as monitored via the harmonic oscillator method of aromaticity index [HOMA]) and the reduced population of the halogen atomic orbitals in the frontier MOs (%XHOMO or %XLUMO). The molecular polarizability and aromaticity, as well as %XHOMO and %XLUMO, are shown to be a function of halogen electronegativity and size, as well as number and position on the ring. It is ultimately demonstrated that halogenobenzenes which are most polarizable and are either least aromatic and/or exhibit the smallest %XLUMO (or largest %XHOMO) values, have the smallest HOMO-LUMO gaps.

Pressure dependence of the forward and backward rates of 9-tert-butylanthracene Dewar isomerization.

9-tert-Butylanthracene undergoes a photochemical reaction to form its strained Dewar isomer, which thermally back-reacts to reform the original molecule. When 9-tert-butylanthracene is dissolved in a polymer host, we find that both the forward and reverse isomerization rates are pressure-dependent. The forward photoreaction rate, which reflects the sum of contributions from photoperoxidation and Dewar isomerization, decreases by a factor of 1000 at high pressure (1.5 GPa). The back-reaction rate, on the other hand, increases by a factor of ∼3 at high pressure. Despite being highly strained and higher volume, the back-reaction reaction rate of the Dewar isomer is at least 100× less sensitive to pressure than that of the bi(anthracene-9,10-dimethylene) photodimer studied previously by our group. These results suggest that the high pressure sensitivity of the bi(anthracene-9,10-dimethylene) photodimer reaction is not just due to the presence of strained four-membered rings but instead relies on the unique molecular geometry of this molecule.

Pressure catalyzed bond dissociation in an anthracene cyclophane photodimer.

The anthracene cyclophane bis-anthracene (BA) can undergo a [4 + 4] photocycloaddition reaction that results in a photodimer with two cyclobutane rings. We find that the subsequent dissociation of the dimer, which involves the rupture of two carbon-carbon bonds, is strongly accelerated by the application of mild pressures. The reaction kinetics of the dimer dissociation in a Zeonex (polycycloolefin) polymer matrix were measured at various pressures and temperatures. Biexponential reaction kinetics were observed for all pressures, consistent with the presence of two different isomers of bis(anthracene). One of the rates showed a strong dependence on pressure, yielding a negative activation volume for the dissociation reaction of ΔV(++) = -16 Å(3). The 93 kJ/mol activation energy for the dissociation reaction at ambient pressure is lowered by more than an order of magnitude from 93 to 7 kJ/mol with the application of modest pressure (0.9 GPa). Both observations are consistent with a transition state that is stabilized at higher pressures, and a mechanism for this is proposed in terms of a two-step process where a flattening of the anthracene rings precedes rupture of the cyclobutane rings. The ability to catalyze covalent bond breakage in isolated small molecules using compressive forces may present opportunities for the development of materials that can be activated by acoustic shock or stress.